Scanned beam displays and image-capture devices have been developed to produce high-resolution images. As shown in FIG. 1, a modulated source 10 of light, which may include a modulated point source of light, is formed into a source beam 13 by optics, e.g., a focus lens 12, and directed by the lens onto a moving scan mirror 14, which reflects the beam onto an image curve or plane, such as a display screen 16, to create a viewable image. The mirror 14 may also scan the beam 13 directly into a viewer's eye (not shown in FIG. 1) to create the image directly on the viewer's retina. One application of this latter technique is for use in a head-worn personal display system, such as described in U.S. Pat. No. 5,467,104, entitled VIRTUAL RETINAL DISPLAY (VRD), which is incorporated by reference. Furthermore, the mirror 14 may be a bi-axial microelectromechanical scanner (MEMS) that scans the beam in a raster pattern. Such a MEMS is described, for example, in U.S. Pat. No. 5,629,790 entitled MICROMACHINED TORSIONAL SCANNER, which is incorporated herein by reference.
Still referring to FIG. 1, an aperture 18 may be placed away from the scan mirror 14, here intermediate the lens 12 and the mirror, to block unwanted light as indicated at 20 and to confine the beam 13 to a clear-optical-quality area of the mirror 14. Ideally, the beam 13 has a cross section equal to the cross section of the clear-optical-quality surface of the scanning mirror 34 at the scanning mirror. Generally, the wider the beam 13 at the scanning mirror 34, the smaller the achievable cross section of the beam at the image display 16, and hence the greater the resolution that the displayed image can have.
Typically, the cross section of the beam is expanded to be no larger than the area of the clear-optical-quality surface of the mirror 34, which is the area of the mirror having high optical quality. This is because the often poor-optical-quality perimeter of the mirror 34 and structure that supports the mirror (e.g., MEMS torsion arms) may scatter or otherwise reflect light from the periphery of the beam 13 (i.e., light that strikes beyond the boundary of the clear-optical-quality surface of the mirror) into the image, and thus create visible image artifacts. Because the mirror-support structures often have surfaces that lie in planes that are parallel to the planes that the mirror 34 moves through, these structures often cause bright spots in portions of the image. For example, if a mirror-support structure is coplanar with the at-rest (i.e., zero) scan position of the mirror 14, then peripheral beam light reflected by the structure may cause the center of the image to appear brighter than the periphery of the image. The perceived brightness of the image is the eyes' integration of the brightness of the scan beam 13 over the area of the image. That is, the scanning of the beam 13 “spreads out” the brightness of the beam over the area of the image. But because light reflected from the mirror-support structures is not scanned, the eyes do not integrate this light over the area of the image. Consequently, this reflected light may cause bright spots in the image. Even a relatively small amount of this unwanted reflected light can cause a visible artifact in the viewed image.
One way to limit or eliminate the peripheral light from the beam 13 that is outside the clear-optical-quality surface of the mirror 14 is to use the aperture plate 18, which is an opaque plate that defines an opening 22 through which the beam 13 propagates. The placement of the aperture plate 18 involves a number of considerations. One placement is as shown in FIG. 1, in the beam path between the lens 12 and mirror 14. The aperture plate 18 thus allows only a certain portion of the beam 13 to propagate through to the mirror. The size of the opening 22, which defines the size of the beam 13, can be calculated based on the distance along the optical path from the source 10 to the mirror 14 and on the focal length of the lens 12 so that the beam 13 fills the optical-quality surface of the mirror 34 completely but does not extend beyond this surface. Although the aperture plate 18 is shown between the mirror 14 and the lens 12, the aperture plate can also be located between the source 10 and the lens.
However, there are imaging systems where the placement of the aperture plate is more constrained. For example, if the system includes multiple sources 10 of light such as in certain types of color display or image capture systems, then the distance that the aperture plate can be from the sources on the source side of the lens 12 is limited by where the light from one source 10 overlaps the light from an adjacent source. That is, light from one source 10 may “leak through” the aperture-plate opening for another source—the aperture plate typically has one opening per source. The phenomenon of light from one source “leaking through” the aperture-plate opening for another source is often called cross talk. Many imaging systems include more than one light source. A color imaging system often uses different colored sources for creating full-color images. Furthermore, some systems include multiple sources of each primary color. For example, such systems may include 13 blue sources 10 to create 13 blue scan beams, 13 red sources 10 to create 13 red scan beams, and 26 green sources 10 to create 26 green scan beams. And these light sources 10 are often LEDs, not lasers. Because many LEDs radiate light over a range of angles, cross talk can be even more of a problem where LEDs are used, and thus aperture-plate placement in such systems may be constrained.
To prevent beam cross talk in scanned-beam systems having multiple sources 10, aperture plates are often placed in one of two locations: 1) close enough to the sources 10 so that light from one source does not propagate through the aperture opening of another source; and 2) close to the scan mirror (much closer than shown in FIG. 1) so that the aperture plate will not interfere with proper beam formation.
Locating an aperture plate close to the sources 10 may require precision in manufacture. For example, the distances between adjacent sources 10 may be on the order of 300 microns, and the diameters of the aperture-plate openings may be on the order of 10 microns. The alignment tolerance of the aperture openings relative to the sources 10 may be on the order of 6 microns. While it may be possible to manufacture an imaging system with such an aperture-plate alignment precision, the cost and difficulty may be prohibitive. Also, such designs may also impose restrictions on design of a corresponding lens assembly.
An aperture plate located close to the scan mirror may reflect the light from the periphery of the beams into the viewer's field of view, and thus cause artifacts in the image as discussed above. In such a location, the aperture plate is or is approximately parallel with the mirror in its rest position. Therefore, as discussed above, the plate may reflect peripheral light from the beams into the center of the image, and thus cause the center of the image to appear brighter than the periphery of the image. Even if the plate has an anti-reflective coating, it may still reflect enough of the peripheral light to create a visible artifact in the image.